Free guanidine is increasingly recognized as a relevant molecule in biological systems. Recently, it was reported that urea carboxylase acts preferentially on guanidine, and consequently, it was considered to participate directly in guanidine biodegradation. Urea carboxylase combines with allophanate hydrolase to comprise the activity of urea amidolyase, an enzyme predominantly found in bacteria and fungi that catalyzes the carboxylation and subsequent hydrolysis of urea to ammonia and carbon dioxide. Here, we demonstrate that urea carboxylase and allophanate hydrolase from Pseudomonas syringae are insufficient to catalyze the decomposition of guanidine. Rather, guanidine is decomposed to ammonia through the combined activities of urea carboxylase, allophanate hydrolase, and two additional proteins of the DUF1989 protein family, expansively annotated as urea carboxylase-associated family proteins. These proteins comprise the subunits of a heterodimeric carboxyguanidine deiminase (CgdAB), which hydrolyzes carboxyguanidine to N-carboxyurea (allophanate). The genes encoding CgdAB colocalize with genes encoding urea carboxylase and allophanate hydrolase. However, 25% of urea carboxylase genes, including all fungal urea amidolyases, do not colocalize with cgdAB. This subset of urea carboxylases correlates with a notable Asp to Asn mutation in the carboxyltransferase active site. Consistent with this observation, we demonstrate that fungal urea amidolyase retains a strong substrate preference for urea. The combined activities of urea carboxylase, carboxyguanidine deiminase and allophanate hydrolase represent a newly recognized pathway for the biodegradation of guanidine. These findings reinforce the relevance of guanidine as a biological metabolite and reveal a broadly distributed group of enzymes that act on guanidine in bacteria.

Heparin-induced thrombocytopenia (HIT) is a common adverse drug reaction associated with frequent life-threatening thrombotic complications. The hallmark of HIT is polyclonal antibodies (Abs) that recognize platelet alpha granule chemokine PF4 when it binds to heparin (PF4/H). These Abs can be detected in solid phase assays that use PF4/H as a target (PF4 ELISA), but only a minority of patients testing positive actually have HIT, i.e., most heparin-induced Abs are non-pathogenic. In patients who have clinical HIT, Abs that activate platelets can be detected using a platelet-activation assay such as the serotonin release assay, or the PF4-dependent p-selectin expression assay (PEA) (Chest 2016; 150:506). Thus, there are at least two distinct types of heparin-induced Abs - those that react only in PF4 ELISA and are seemingly "non-platelet-activating" and "non-pathogenic" and those that are "platelet-activating" and "pathogenic". To date, the molecular basis for the differing clinical and serologic behaviors of pathogenic and non-pathogenic Abs is uncertain. To address this issue, we performed single cell cloning to clone B cell receptors from IgG1+ B cells from HIT patients. We deposited single B cells (CD19+IgG1+) from 6 patients with "classical" and 2 patients with "spontaneous" HIT into 96 well plates containing feeder cells (from G Kelsoe, Duke U) that support B cell proliferation and Ab secretion (Immunity 2018;48:174). Clones secreting IgG were first screened in PF4 ELISA and positive results were obtained with 55 clones from 6 patients. Further screening showed that 7 of these clones (from 4 patients) were also PEA-positive (platelet-activating). Clones positive only in PF4 ELISA, positive in both PF4 ELISA and PEA, or negative in PF4 ELISA were designated NP (non-pathogenic), PA (platelet-activating) and NB (non-binding), respectively. H and L chain variable regions were defined in 7 PA, 42 NP and 34 NB clones. The following findings were made when sequences in the 3 clonal groups were compared: PA clones preferentially used JH6 (p=0.002) and the VH3/JH6 combination (p=0.0003)The PA and NP Abs all employed κ chains, whereas κ chain usage for NB clones was 61% (p<0.0001).No preferred signatures were identified in κ chain complementarity determining regions (LCDR3) of PA clones that differentiate them from NP and NB Abs.PA Abs had longer heavy chain CDR3s (HCDR3) than NP (p<0.001) or NB (p=0.0001) AbsPA Abs contained more positively charged amino acid residues compared to NP (p=0.058) or NB (p=0.002) Abs.PA Abs contained more tyrosine residues compared to NP (p=0.067) or NB (p<0.0001) AbsFive of 7 PA clones contained an RX1-2K/RX1-2R/H (RKH) motif in HCDR3; the remaining 2 PA clones contained a string of at least 5 tyrosines (Y5 motif) in HCDR3. The RKH and Y5 motifs were not found in any of the 76 NP and NB clones. Substitution of alanine for positively charged residues of the RKH motif or of tyrosine residues in the Y5 motif in PA clones reduced PF4/H binding and platelet activation, arguing for functional significance of both motifs. Utilization of nearly identical H and L chains within 3 groups of clones and of shared H chains within 3 groups of clones (both PA and NP) was observed in multiple patients. Moreover, utilization of a shared H chain was observed within 3 NP clones from two unrelated patients. These findings indicate clonal amplification and convergence of the B cell (both PA and NP) response, likely in response to a common antigen. High throughput sequencing of IgG H chains were performed on peripheral blood mononuclear cells (PBMC) from 7 HIT patients and 3 healthy donors. Eleven of 1585 H chain sequences (0.69%) from HIT patients contained the RKH and 18 (1.1%) contained the Y5 motif. In 3 healthy donors, 4 of 1418 H chain sequences (0.28%) contained RKH and none (0%) contained Y5. The findings reflect amplification of B cells with receptors containing RKH and Y5 motifs in HIT patients (p=0.1 for excess RKH and p<0.0001 for Y5 in HIT). These observations provide the first characterization of Ig structural motifs that are favored for selection in the humoral immune response leading to HIT and suggest that the RKH and Y5 CDR3 motifs in particular may contribute importantly to Ab pathogenicity. Findings made are expected to facilitate further work to define features specific to "pathogenic" HIT Abs and, possibly, to identify genetic variants that predispose individuals to experience HIT. Disclosures Padmanabhan: Terumo BCT: Consultancy; Veralox Therapeutics: Membership on an entity's Board of Directors or advisory committees; Versiti Wisconsin: Patents & Royalties: Related to HIT patents; Retham Technologies: Equity Ownership; Janssen R&D: Consultancy.

Heparin-induced thrombocytopenia (HIT) is a serious adverse drug reaction characterized by antibodies that recognize platelet factor 4/heparin complexes (PF4/H) and activate platelets to create a pro-thrombotic state. While a high percentage of heparin-treated patients produce antibodies to PF4/H, only a subset also makes antibodies that are platelet-activating. A close correlation between platelet-activating antibodies and the likelihood of experiencing HIT has been demonstrated in clinical studies but how platelet-activating (presumptively pathogenic) and non-activating (presumptively benign) antibodies differ from each other at the molecular level is unknown. To address this issue, we cloned seven platelet-activating (PA) and 47 non-activating (NA) PF4/H-binding antibodies from six HIT patients and characterized their structural and functional properties. Findings made showed that PA clones differed significantly from NA clones in possessing one of two heavy chain complementarity-determining region 3 (HCDR3) motifs - RX1-2R/KX1-2R/H (RKH) and YYYYY (Y5) - in an unusually long CDR3 region (≥20 residues). Mutagenic studies showed that modification of either motif in PA clones reduced or abolished their platelet-activating activity and that appropriate amino acid substitutions in HCDR3 of NA clones can cause them to become platelet-activating. Repertoire sequencing showed that the frequency of peripheral blood IgG+ B cells possessing RKH or Y5 was significantly higher in HIT than in non-HIT patients given heparin, indicating expansion of B cells possessing RKH or Y5 in HIT. These findings imply that antibodies possessing RKH or Y5 are relevant to HIT pathogenesis and suggest new approaches to diagnosis and treatment of this condition.

Urea amidolyase (UAL) is a biotin‐dependent enzyme composed of urea carboxylase (UC) and allophanate hydrolase (AH). UAL catalyzes the cleavage of urea into ammonia and CO2 through an ATP and HCO3‐ ‐dependent reaction. Curiously, many bacteria also have the enzyme urease which also can degrade urea, but through an ATP‐independent reaction. An explanation for this conundrum has remained elusive, but we propose a solution. It has recently been reported that the expression of bacterial UC can be regulated by a guanidine‐sensitive riboswitch. Additionally, bacterial UC has a catalytic efficiency of ATP cleavage that is ~40‐fold higher in the presence of guanidine compared to urea. This suggests that guanidine is a preferred substrate for UC and that guanidine may be degraded into ammonia and CO2 by a similar pathway to urea. Here, we confirm that the ATP cleavage is accelerated in the presence of guanidine compared to urea. However, we did not observe the complete degradation of guanidine to ammonia and CO2 through the combined actions of UC and AH. The genes of UC and AH are often clustered with two genes of unknown function, annotated as urea carboxylase‐associated family proteins. We reveal that these genes encode the subunits of a heteromeric carboxyguanidine deiminase (CgdAB), which is required for the complete degradation of guanidine to ammonia and CO2 in the presence of UC and AH. We propose that the guanidine degradation pathway includes carboxylation of guanidine to form carboxyguanidine by UC, deimination of carboxyguanidine to form allophanate by CgdAB, and hydrolysis of allophanate to ammonia and CO2 by AH. Interestingly, we reveal that the fungal UAL enzymes of Candida albicans and Saccharomyces cerevisiae are unable to degrade guanidine in the presence of CgdAB and do not use guanidine as a substrate. We show that 75% of bacterial UC genes do colocalize with a cgdAB gene, and all fungal UAL genes do not. A subtle enzymatic difference of the fungal UALs and the bacterial UCs is the presence of an Asp/Asn residue mutation in the carboxyltransferase active site. Additionally, most of the 25% of bacterial UC enzymes whose genes do not colocalize with cgdAB also contain an Asn, consistent with the variation observed in fungal UALs. The exclusive presence of CgdAB in bacteria suggests that a mechanism for metabolic guanidine degradation has independently evolved in the bacteria relative to the fungi.

A new Dethiosulfovibrio strain, designated F2BT, was isolated from an anaerobic digester for treating solid waste from a marine recirculating aquaculture system. The motile, Gram-negative, non-spore-forming curved rods were 2–7 µm long and 1 µm in diameter. Growth occurred at temperatures ranging from 20 to 40 °C with a maximum rate of growth at 30 °C. The pH range for growth was pH 6.0–8.0, with a maximum rate of growth at pH 7.5. This isolate was halotolerant growing in NaCl concentrations ranging from 0 to 1.6 M with a maximum rate of growth at 0.4 M. Similarly to the five described Dethiosulfovibrio species, this obligate anaerobe isolate was fermentative, capable of utilizing peptides, amino acids and some organic acids for growth, but unlike described strains in the genus did not reduce thiosulphate or elemental sulphur to hydrogen sulphide during fermentation of organic substrates. The G+C content of 55 mol% is similar to the described Dethiosulfovibrio species. The average nucleotide identity analysis between whole genome sequences showed less than 93.15% sequence similarity between strain F2BT and the five other described Dethiosulfovibrio species. Differences in the physiological and phylogenetic characteristics between the new strain and other Dethiosulfovibrio specied indicate that F2BT represents a novel species of this genus and the epithet Dethiosulfovibrio faecalis sp. nov. is proposed. The type strain is F2BT (=DSM 112078T=KCTC25378T).